The vitamin A metabolite, retinoic acid, has long been recognized to induce a broad spectrum of biological effects. For example, retinoic acid-containing products, such as Retin-A® and Accutane®, have found utility as therapeutic agents for the treatment of various pathological conditions. In addition, a variety of structural analogues of retinoic acid have been synthesized that also have been found to be bioactive. Many of these synthetic retinoids have been found to mimic many of the pharmacological actions of retinoic acid, and thus have therapeutic potential for the treatment of numerous disease states.
Medical professionals have become very interested in the therapeutic applications of retinoids. Among their uses approved by the FDA is the treatment of severe forms of acne and psoriasis as well as cancers such as Kaposi's Sarcoma. A large body of evidence also exists that these compounds can be used to arrest and, to an extent, reverse the effects of skin damage arising from prolonged exposure to the sun. Other evidence exists that these compounds have clear effects on cellular proliferation, differentiation and programmed cell death (apoptosis), and thus, may be useful in the treatment and prevention of a variety of cancerous and pre-cancerous conditions, such as acute promyleocytic leukemia (APL), epithelial cancers, squamous cell carcinomas, including cervical and skin cancers and renal cell carcinoma. Furthermore, retinoids may have beneficial activity in treating and preventing diseases of the eye, cardiovascular disease and other skin disorders.
Major insight into the molecular mechanism of retinoic acid signal transduction was gained in 1988, when a member of the steroid/thyroid hormone intracellular receptor superfamily was shown to transduce a retinoic acid signal. V. Giguere et al., Nature, 330:624-29 (1987); M. Petkovich et al., Nature, 330: 444-50 (1987); for a review, see R. M. Evans, Science, 240:889-95 (1988). It is now known that retinoids regulate the activity of two distinct intracellular receptor subfamilies: the Retinoic Acid Receptors (RARs) and the Retinoid X Receptors (RXRs), including their subtypes, RARα, β, γ and RXRα, β, γ. All-trans-retinoic acid (ATRA) is an endogenous low-molecular-weight ligand that modulates the transcriptional activity of the RARs, while 9-cis retinoic acid (9-cis) is the endogenous ligand for the RXRs. R. A. Heyman et al., Cell, 68:397406 (1992); and A. A. Levin et al., Nature, 355:359-61 (1992).
Although both the RARs and RXRs respond to ATRA in vivo, due to the in vivo conversion of some of the ATRA to 9-cis, the receptors differ in several important aspects. First, the RARs and RXRs are significantly divergent in primary structure (e.g., the ligand binding domains of RARα and RXRα have only approximately 30% amino acid homology). These structural differences are reflected in the different relative degrees of responsiveness of RARs and RXRs to various vitamin A metabolites and synthetic retinoids. In addition, distinctly different patterns of tissue distribution are seen for RARs and RXRs. For example, RXRα mRNA is expressed at high levels in the visceral tissues, e.g. liver, kidney, lung, muscle and intestine, while RARα mRNA is not. Finally, the RARs and RXRs have different target gene specificity.
RARs and RXRs regulate transcription by binding to response elements in target genes that generally consist of two direct repeat half-sites of the consensus sequence AGGTCA. It is believed that RAR operates predominantly through a heterodimer complex with RXR. RAR:RXR heterodimers activate transcription by binding to direct repeats spaced by five base pairs (a DR5) or by two base pairs (a DR2). RXRs can also form homodimers. RXR:RXR homodimers bind to a direct repeat with a spacing of one nucleotide (a DR1). D. J. Mangelsdorf et al., “The Retinoid Receptors” in The Retinoids: Biology, Chemistry and Medicine, M. B. Spom, A. B. Roberts and D. S. Goodman, Eds., Raven Press, New York, N.Y., 2nd Edition (1994). For example, response elements have been identified in the cellular retinal binding protein type II (CRBPII), which consists of a DR1, and in Apolipoprotein AI genes that confer responsiveness to RXR, but not to RAR. Further, RAR has also been shown to repress RXR-mediated activation through the CRBPII RXR response element (D. J. Manglesdorf et al., Cell, 66:555-61 (1991)). RXRs, however, act predominantly as coregulators, which enhance the binding of all-trans retinoic acid, vitamin D3, thyroid hormone, and peroxisome proliferator-activated receptors to their response elements through heterodimerization. Also, RAR specific target genes have been identified, including target genes specific for RARβ (e.g., βRE), that consist of a DR5. These data indicate that two retinoic acid responsive pathways are not simply redundant, but instead manifest a complex interplay.
RXR agonists in the context of an RXR:RXR homodimer display unique transcriptional activity in contrast to the activity of the same compounds through an RXR heterodimer. Activation of a RXR homodimer is a ligand dependent event, i.e., the RXR agonist must be present to bring about the activation of the RXR homodimer. In contrast, RXR working through a heterodimer (e.g., RXR:RAR, RXR:VDR) is often the silent partner, i.e., no RXR agonist will activate the RXR-containing heterodimer without the corresponding ligand for the heterodimeric partner. However, for other heterodimers, (e.g., PPAR:RXR) a ligand for either or both of the heterodimer partners can activate the heterodimeric complex. Furthermore, in some instances, the presence of both an RXR agonist and the agonist for the other heterodimeric partner (e.g., gemfibrizol for PPARα and TTNPB for RARα) leads to at least an additive, and often a synergistic enhancement of the activation pathway of the other IR of the heterodimer pair (e.g., the PPARα pathway). See e.g., WO 94/15902, published Jul. 21, 1994; R. Mukherjee et al., J. Steroid Biochem. Molec. Biol., 51:157-166 (1994); and L. Jow and R. Mukherjee, J. Biol. Chem., 270:3836-40 (1995).
RXR modulators which have been identified so far have exhibited significant therapeutic utility, but they have also exhibited some undesirable side effects. For instance, retinoids have been shown to elevate triglycerides and suppress the thyroid hormone axis (see, e.g., Sherman, S. I. et al., N. Engl. J. Med. 340(14):1075-1079 (1999). In addition, many retinoids have undesirable side effects such as skin irritation, lipid and bone toxicity, visual effects (including night blindness and dry eye) and teratogenicity. Therefore, development of new compounds that modulate RXR homo- and heterodimer activity while exhibiting fewer side effects is desirable.